1. Field of the Invention
The present invention relates to improved methods and apparatus for the filling of acetylene gas cylinders.
2. Prior Art
As is well known, acetylene gas is relatively unstable at high pressures and cannot be transported safely in such open-chambered cylinders as are used to transport other industrial gases. For safety reasons, acetylene is usually transported in elongate steel cylinders of a specialized type, each containing a porous mass within which a solvent for acetylene is absorbed. The porous mass normally fills the cylinder body, and typically comprises a very porous, concrete-like substance such as calcium metasilicate having an admixture of a suitable fibrous material, for example asbestos, to increase its mechanical strength. The solvent is typically acetone or N,N-dimethylformamide.
From the point of view of safety it is important that the porous mass which fills the body of an acetylene cylinder leave no large cavities within the body wherein acetylene gas can collect and be compressed, causing an explosive decomposition of acetylene to take place. Decomposition of acetylene into its elements can be prevented by minimizing the size of the spaces defined (1) within the porous mass, and (2) between the porous mass and the surrounding internal wall surfaces of the cylinder.
The filling of acetylene cylinders involves problems beyond those normally encountered in filling cylinders with gases other than acetylene. When an acetylene cylinder is returned from a customer, it contains an unknown quantity of residual acetylene gas, and an unknown quantity of solvent. The quantity of solvent remaining in the cylinder is almost always less than the desired nominal amount due to discharge of evaporated solvent from the cylinder as acetylene gas is used by the customer.
The only information which can be ascertained about a returned cylinder by physically inspecting it is whether or not the cylinder appears to be damaged, and such cylinder data as is stamped or otherwise inscribed on the outside of the or on a cap affixed to the cylinder. The cylinder data normally includes a cylinder type designation (which defines the cylinder's internal volume), and the cylinder's "tare weight," i.e., weight of the cylinder together with the weight of the porous mass and the weight of a proper nominal charge of acetone solvent. By weighing a returned cylinder, it is possible to determine the total weight of the cylinder and such solvent and residual gas as remain in the cylinder. By measuring the pressure of the contents of the cylinder, and by taking into account the temperature of the cylinder, the exact quantities of residual gas and solvent which are present in the cylinder can be calculated readily.
In a conventional acetylene cylinder filling process, each cylinder is first physically inspected for possible damage. The contents of the cylinder are then checked with reference to the cylinder's tare weight, pressure and temperature, to determine the quantities of solvent and gas which remain in the cylinder. A calculation is then made to determine the quantities of solvent and gas which should be supplied to refill the cylinder.
In accordance with conventional filling procedures, an acetylene cylinder which has been checked in the manner described above is refilled first by charging it with the requisite amount of missing solvent, and then by introducing the requisite quantity of acetylene gas. The gas is fed to the cylinder at a relatively low pressure during a period of time which extends for many hours. While cylinders are charged on an individual basis to supply them with the requisite quantities of solvent, a plurality of solvent-replenished cylinders are connected together or "ganged" for simultaneous filling with acetylene.
While a cylinder is being filled with acetylene, its temperature is caused to rise due to the high heat of solution of acetylene. Stated in another way, as acetylene gas is dissolved by a cylinder-carried solvent such as acetone, a substantial amount of heat energy is released, causing a marked elevation in the temperature of the cylinder and its contents. As the temperature of the cylinder's contents rises, so too does the pressure within the cylinder. When the pressure within the ganged cylinders reaches approximately 25 bar (about 360 psia), filling is halted because, from a safety point of view, this is considered to be about the highest pressure to which acetylene should normally be compressed. The partially filled cylinders are then left standing to cool.
After a sufficient period of cooling time, the cylinders stabilize in temperature, and can then be further charged with acetylene to complete the filling procedure. The filled cylinders are then disconnected from the filling apparatus and individually weighed to make certain that they contain, within certain tolerances, the prescribed quantity of acetylene. Any cylinders showing excess weight are slightly emptied. Any cylinders which have been insufficiently filled are given an additional filling.
In temperate climates, about seven hours is a normal time for the initial filling of a solvent-charged cylinder with acetylene gas, followed by about a twelve hour pause for cooling, whereafter a final filling with acetylene usually requires about an additional two hours. In hotter climates these filling and cooling times are considerably longer.
In order to diminish filling time, it has been proposed to cool acetylene cylinders during filling by spraying their outer walls with a liquid coolant. The coolant is discharged onto the cylinders from overhead nozzles, and typically comprises either cold water, or a cold antifreeze solution such as a mixture of water and alcohol. The overhead arrangement of spray nozzles causes coolant to flow along the full lengths of the outer walls of the cylinders. Cooling the cylinders during filling not only serves to reduce cylinder filling time, but also enables larger numbers of cylinders to be processed through a filling station, and minimizes the need for extensive banks of filling equipment. Moreover, the cost of labor per filled cylinder is reduced.
3. The Mechanical System Proposal
In order to provide for rapid filling of acetylene cylinders, a mechanical system, hereinafter referred to as the "Mechanical System," has been proposed consisting basically of a relatively complex scale having a programmable weight level which, when reached, will cause the system to transfer from charging a cylinder with acetone to charging the cylinder with acetylene. In utilizing this proposal, an operator positions a cylinder on the scale, reads the tare weight of the cylinder, and inputs that information to the scale together with information regarding the pressure and temperature of the cylinder's contents. Based on this information, a calculation is made as to the quantity of solvent which should be added during filling. The process of replenishing the cylinder with solvent and gas then proceeds, monitored by the scale. As with conventional filling processes, the total required quantity of solvent is replenished first. Only after the entire requisite charge of solvent has been introduced into the cylinder does acetylene filling begin. Acetylene filling continues until such time as a predetermined weight is sensed by the scale, whereupon filling is halted, and the filled cylinder is removed from the scale.
A problem encountered both with conventional filling techniques and with the Mechanical System proposal is the possibility, if not the likelihood, that cylinders will, when filled, contain a higher than desired weight of acetylene, and a lower than desired weight of acetone. Such a situation is undesirable, not only from the point of view of safety, but also from the point of view of cost. Acetylene is far more expensive per pound than is acetone.
The Mechanical System proposal has limitations in accuracy and programmability. By virtue of its mechanical nature, this proposal is limited in use to a weight-monitored filling process wherein the missing weight of solvent is first totally replenished in an initial filling step, whereafter the requisite weight of acetylene supplied in a final filling step. The Mechanical System proposal does not fully address many safety considerations which are desirably taken into account in the rapid filling of acetylene cylinders, and can, in the event of a weight-sensing malfunction, permit a cylinder to be overfilled.
As will be apparent from the description of the present invention which follows, what the present invention has in common with the Mechanical System proposal is individual, rapid, high-pressure filling of cylinders with acetylene while the cylinders are each positioned atop a separate weight-monitoring device, and while outer wall portions of the cylinders are being cooled with a flow of coolant. However, as also will be apparent from the discussion which follows, the system of the present invention provides many advantages over the Mechanical System proposal, including significant advances which enhance the safety and efficiency with which acetylene cylinders are filled.
4. The Referenced Applications
The referenced Fixed Cap Case relates to a protective cap for a pressurized gas cylinder. The cap has cylinder data encoded on it. The Fixed Cap Case also relates to a method and apparatus for reading encoded cylinder data for automatically inputting the data to a control unit. Use is made of the invention described in the Fixed Cap Case in carrying out the preferred practice of the present invention.
The referenced Cooling System Case addresses a problem which has not been recognized previously, namely that the conventional practice of cooling cylinders along the full lengths of their outer walls during filling with acetylene does not provide, from the point of view of safety, an optimum type of cooling. The invention of the Cooling System Case provides for selective cooling of only lower portions of the outer wall of an acetylene cylinder during filling to achieve a highly desirable, non-uniform concentration of dissolved acetylene within a cylinder being filled, with the lowest concentration of dissolved acetylene being in the upper end region of the cylinder, whereby the cylinder's resistance to backfire is significantly enhanced. Use is made of the invention described in the Cooling System Case in carrying out the preferred practice of the present invention.